In the case of a blood sugar measurement carried out by a patient himself or herself, the patient supplies a drop of blood to the test field of a test strip which is then optically measured, with a detector for example capturing the change in color of the measuring field, which change in color is evoked by the application of the blood to the test field. The signal given by the detector corresponds to an average value of the color change in the test field. A correct value is then only obtained if the amount of blood is sufficient to uniformly wet the entire measuring field. On the other hand, if the measuring field is only partially wetted, the measurement is falsified in that in the communication of the measured value areas are included in which practically no chemical reaction has taken place and accordingly no color change has occurred. By the time this fault is recognized in a customary measurement as a rule no subsequent dosing is any longer possible, because in the wetted portion of the measuring field the chemical reaction already has advanced too far and even with a further dropping of blood a uniform discoloration of the measuring field can no longer be obtained. Moreover, as rule on the part of the patient the small puncture wound out of which the blood drop has been pressed has again so far closed that no further blood escapes. The patient must therefore repeat the measurement, which for him or her is very unpleasant when one remembers that a patient, depending on circumstances, may have to carry out such measurement several times a day and each time has to stick himself or herself in a finger.